dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Oct 20, 2009 at 02:30:56PM +0200, Johan Hake wrote:
> On Tuesday 20 October 2009 13:57:16 Anders Logg wrote:
> > Here's an issue I've run into lately.
> >
> > The current design of the Expression class forces the specification
> > of a FunctionSpace or UFL FiniteElement when one creates an Expression:
> >
> >   v = Expression("sin(x[0])", V=V)  or
> >   v = Expression("sin(x[0])", element=element)
> >
> > This is only necessary in Python, but not in C++ where it's enough to
> > set the geometric dimension.
>
> All you ask for is already here for you, but not in a shiny easy accessible
> way.
>
>   CompiledClass = compile_expressions(["sin(x[0])"],[{}])[0]
>   expr = CompiledClass(2)
>
> This is a cpp.Expression, which you cannot use in a ufl form and it does not
> have the nice __call__ method.
>
> > Would it be possible to add an optional constructor that just takes
> > the geometric dimension as an argument?
>
> This is what the above do.
>
> > I'm writing some code where I create various Expressions and project
> > to different function spaces (for some theoretical studies related to
> > adaptivity). Then it shouldn't matter (and it doesn't) which function
> > space or element I originally specify in the constructor of my sin(x)
> > expression.
> >
> > If necessary, we can automatically create either a piecewise linear
> > Lagrange element based on the incoming geometric dimension. Another
> > option would be to use a QuadratureElement.
>
> I will not make this shine for now. However, the future CompileExpression
> could take an argument dim, that if given, will create a cpp.Expression only
> class which cannot be used in ufl.forms.

ok.

> I also suggest that we keep the future dolfin.Expression class as a combined
> cpp.Expression/ufl.Function class. If not we could add a new class called
> FormExpression which has the dual inheritance, and Expression which
> essentially just is cpp.Expression.

Agree.

--
Anders

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